Patent GB-1229864-A (GB)
IPC: H04Q11/04
Filed: 1968-03-19
Abstract: 1,229,864. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO. Ltd. 12 March, 1969 [19 March, 1968], No. 13299/68. Heading H4K. The Specification describes a transit exchange switching between T.D.M. highways, each employing P.C.M. transmission characterized in that:-(a) each P.C.M. sample comprises n bits (i.e. n - 1 speech bits and a signalling bit indicative of the state of a line) transmitted over a highway in serial form. (b) The highways are grouped in groups of n, each group being connected to the transit exchange via an n wire superhighway over which the bits of each sample are passed in parallel (one bit per wire). (c) Each superhighway can be connected via a switching matrix to any one of a number of "cord" circuits. (d) Each cord circuit comprises a discrete storage location for each time slot on a superhighway. (e) Each location comprises at least five sections for storing the n bits of a sample, the address of the crosspoint (of the switching matrix) which must be operated to connect to the originating channel, a similar address for the terminating channel, a code indicative of the point reached in the processing of that call and the address of another storage location and to which access is required next and (f) means which during selected processing cycles tests the signalling bit to determine the state of an originating or terminating channel. In the preferred form four-wire transmission is employed one pair of wires of each set conveying information to the exchange over a receive highway and the other pair conveying information from the exchange over the transmit highway. Each P.C.M. sample comprises seven speech bits and a signalling bit and each highway carries 12 originating and 12 terminating channels using 192 time slots. Each superhighway carries 192 channels (96 originating and 96 terminating) in the same 192 time slots. 4 superhighways and 3 cord circuits are employed. Cord circuits.-Each cord circuit includes a magnetic core store having an address register providing random access and input and output buffers. The locations in the store are organized as shown. Each location or row stores the signalling bit SB, the speech bits or (during dialling) a register address SPB/RA, the cross-points address associated with the originating channel CCA, the crosspoint address associated with the terminating channel ACA, the location to be accessed next TSA, a code indicative of the state reached in setting up the call CS, and 4-bit locations used to form the registers. Call processing.-Processing generally proceeds on a 192 bit cycle or frame synchronized with the 192 time slot frame of each superhighway, eight such frames being referred to as a superframe. During each frame each location of each cord is accessed twice, i.e. during odd numbered time slots the 96 locations are accessed sequentially in numerical order while during an odd-numbered time slot the time slot having an address specified in section TSA of the location accessed during the preceding oddnumbered time slot. During each superframe each of the three cord circuits is associated with a different one of the four superhighways, the four superhighways being selected cyclically in threes during four successive superframes for this purpose. During each superframe each cord circuit scans the associated superhighway for new calls, a duration test covering a plurality of frames being necessary as the signalling bit is used alternately for line condition and synchronizing signals, i.e. for each odd-numbered time slot in which the associated storage location stores at CS a code (e.g. zero) indicating that that cord is not otherwise in use during that slot, the signalling bit is tested for a "1" condition. If such a "1" condition is detected during the first or third frame of a superframe the code CS is changed to "seize suspected" or "one". If the condition persists during the fifth frame and at least one of the eight registers associated with the cord is free a new call is recognized and the code CS is changed to "register allocated" or "two". A counter is cycled to generate successive register addresses and when a free register is found its address is written at SPB/RA in the location associated with the originating channel causing that register (say register 1) to be connected to the originating channel. When sufficient dialling pulses have been received in the register they are read out and translated to give the address of the wanted channel or channels. During each even numbered time slot associated with a wanted channel that channel is tested for availability using its busy wire and an indication of its state forwarded to the central control. The central control receives signals from all cords and selects a cord and location to be used for a call. The addresses CCA and ACA of the cross-points associated with the originating and terminating channels respectively are recorded in that location of the selected cord which is accessed in the time slot of the originating channel and the address of this location is recorded at TSA of the location accessed immediately prior to the time slot of the terminating channel. Thus during each frame the former location is accessed twice once, in the time slot of the originating channel and once in the time slot of the terminating channel. During the first such access a coded speech sample from SPB/RA is sent to the originating channel over the transmit highway and a coded speech sample from the receive highway of the originating channel is entered into SPB/RA while during the other such access this coded sample is fed to the transmit highway of the terminating channel and a coded sample from the receive highway of that channel is entered into SPB/RA. In this way the coded speech samples are passed in both directions between the channels using a single storage location.